The present invention relates generally to the release of pressure sensitive adhesives, such as organic pressure sensitive adhesives and silicone pressure sensitive adhesives, herein also called PSAs, OPSAs and SPSAs, respectively. In a particular aspect, this invention relates to a method for coating a substrate to provide a release backing which can be used to protect SPSAs, to curable coating compositions therefor and to articles of manufacture comprising the cured coatings.
SPSAs, such as those disclosed in U.S. Pat. Nos. 2,736,721; 2,814,601; 2,857,356; 3,528,940 3,929,704; 3,983,298; 4,309,520; Canadian Patent No. 711,756 and British Patent No. 998,232 are well known for their excellent thermal stability and tenacious adhesiveness. While these properties have valuable utility the adhesiveness, i.e. tack and/or adhesive strength, of SPSAs is also a significant problem.
Whether the SPSA has the form of, for example, an adhesive layer on substrate in the form of a roll of tape; an adhesive layer on an article protected with a peelable backing, such as a decorative trim item to be adhered to an automobile or a medical item to be adhered to the skin of a person; or a transfer adhesive layer protected on two surfaces by peelable backings, the SPSA must be separated from an adjacent surface before it can be used for its intended purpose.
Because of the above-noted and well-known tenacious adhesiveness of SPSAs substantially every material that has been used as a protective backing or as a tape substrate for SPSAs has been given some sort of surface treatment to facilitate the removing of the adjacent surface from the adhesive without adhesive separation or transfer and with a force sufficiently small to avoid the tearing of the substrate, item or backing.
Keil, U.S. Pat. No. 3,050,411, employed a dispersion of a mixture of a methylhydrogenpolysiloxane, certain fluoroalkyl-substituted siloxanes and a curing catalyst as a surface release agent. The siloxanes had a viscosity of at least 5000 centistokes at 25.degree. C and consisted of at least 90 mol % of fluorinated siloxane units having the formula RCH.sub.2 CH.sub.2 Si(CH.sub.3)O, wherein R is a perfluoroalkyl group having less than 4 carbon atoms, not more than 10 mol % of siloxane units having the formula R'.sub.n SiO.sub.(4-n)/2, a degree of substitution ranging from 1.9 to 2.0 and at least two silicon-bonded hydroxyl or lower alkoxyl radicals. Keil's compositions were found to release SPSA tapes with ease and without loss of the adhesiveness of the SPSA; however, said compositions wherein R is CF.sub.3 do not provide easy release of SPSAs which have been cast thereon from a solvent solution of the adhesive. In addition, said compositions do not provide suitable release of curable SPSAs that have been cast and cured thereon. Compositions wherein R was C.sub.2 F.sub.5 or a mixture of CF.sub.3 and C.sub.3 F.sub.7 were said to give similar results.
O'Malley, U.S. Pat. No. 4,039,707, noted that if the SPSA was of a certain type, i.e. a SPSA containing diphenylsiloxane units, the standard release coatings based on dimethylsiloxanes that were used with OPSAs were suitable for use therewith as a release composition. However, it is known that dimethylsilicone coatings will not release dimethylsiloxane-based SPSAs.
Olson, U.S. Pat. No. 4,472,480, proposed a release backing comprising an insoluble polymer film having a plurality of perfluoroalkylene oxide repeating units. When formed by in-situ polymerization of a monomer solution the polymer film was said to resist transfer to aggressively tacky PSAs and to be exceedingly thin. Among the polymerizable monomers illustrated by Olson were monomers bearing acrylate, epoxy, isocyanate and hydrolyzable silane groups. When epoxy-containing monomers were used epoxy-substituted silanes could be copolymerized therewith. When monomers bearing hydrolyzable silane groups were used "silanes which may be linear or cyclic" were said to be copolymerizable therewith.
Koshar, European Patent Application No. 165,059, dated Dec. 18, 1985, discloses a low energy release liner for SPSAs comprising the hydrosilylation reaction product of an ethylenically unsaturated perfluoropolyether and a compound bearing silicon-bonded hydrogen atoms.
It is apparent from the above that the preparation of a completely acceptable release backing for SPSAs is a long-lived problem that continues to command research and development resources.
One reason for the continuing research and development on release coatings for SPSAs is that the preferred process for preparing an article containing a PSA and a release backing, which process comprises casting a solution of the PSA onto the release backing and then bonding the article to the adhesive layer, aggravates the subsequent release of a SPSA from the release backing to the extent that an unacceptably high release force is needed to separate the release backing from the adhesive and/or the SPSA has an unacceptably low adhesive force after it has been separated from the release backing.
The use of this casting practice, instead of applying the release backing to an adhesive layer already formed on the article, is necessary, for example, in the process of forming an adhesive transfer tape. In this process the PSA is cast onto one release backing and a second release backing is thereafter applied to the adhesive layer. Although it is necessary that the adhesive release from one of the release backings more easily than from the other release backing, the greater release force must not be so great as to result in cohesive failure of the adhesive or tearing of the release backing.
The use of this casting practice is also necessary, for example, when an article to which a heat-curable PSA is bonded is sensitive to the temperatures used in the curing process. In this case the PSA is cast onto the release backing and heat-cured and then the heat-sensitive article is bonded to the adhesive layer.
Until the present invention there had been no release coating compositions that would release solvent-cast, curable SPSAs with acceptable release force and without substantially altering the adhesiveness of the released SPSA.
Another reason for the continuing research and development on release coatings for SPSAs is the progress in the formulation of SPSAs. For example, curable SPSAs having an adhesive strength substantially exceeding 1,200 grams/inch, as measured by standard methods hereinafter described, are now available. For another example, recently developed SPSAs that have resistance to amine-containing materials, such as medicines, present new release problems, even though they require release forces much less than 1,000 grams/inch. In this regard, reference is made to U.S. patent application Ser. Nos. 665,796; 665,797 and 665,805, filed on Oct. 29, 1984 an 780,505, filed on Aug. 26, 1985, said applications being assigned to the assignee of the present application.
An improved release backing for SPSAs, particularly solvent-cast SPSAs, and most importantly for solvent-cast, heat-curable SPSAs, is thus needed in the adhesives industry.